1. Field of the Invention
The invention relates to test equipment, particularly with respect to measuring depth of modulation of frequency modulated signals.
2. Description of the Prior Art
An a.c. carrier signal that is frequency modulated by, for example, a sinusoidal modulating signal is often provided by a waveform generator for utilization as a test signal or a stimulating signal in commercial and military environments. Such uses include commercial communication networks and military equipment such as sonar systems. It is often a requirement in such applications that the depth of modulation be measured to a high degree of accuracy such as 0.1% or better. Depth of modulation is the maximum positive and negative frequency deviation of the carrier signal as a result of the frequency modulation. In many commercial applications, the carrier frequencies utilized are in the radio, video and microwave ranges. In certain other applications, such as in sonar, the carrier or fundamental frequency may be in the audio range such as under 30 KHz. Precisely accurate measurements of the depth of modulation may be required, for example, for adjusting the carrier or the modulating signal of such waveform generators to compensate to anomolies such as drift.
Present day commercially available frequency modulation testers provide accuracies of only 2% to 3% of indicated readings. Additionally, testers are not available to obtain measurements on fm signals with fundamental frequencies below 100 KHz. Such present day fm testers are predicated on the principle of designing the best possible fm receiver with the best possible fm discriminators to provide the highest possible accuracy in the depth of modulation measurement. Such testers demodulate the incoming waveform to provide the depth of modulation. As discussed above, such testers do not achieve greater accuracy than 2% to 3% and cannot be utilized with carrier frequencies below 100 KHz. Additionally, such test instrumentation tends to be expensive since fm discriminators even with the limited accuracy available tend to be undesirably costly.